1. Field of the Invention
The present invention relates to a current switching circuit and particularly to a current switching circuit capable of high-speed switching operation and suitable for connection with a current driving element such as a laser diode.
2. Description of Related Art
A laser diode is widely used as a light source in optical information processing units. For example, a laser diode is used as a light source of an optical head in an optical disc device. In the laser diode, output light is turned ON/OFF by a current switching circuit. For the current switching circuit, the high-speed switching operation is demanded to achieve higher information processing, and the control of output current is demanded to provide an appropriate output from the laser diode according to usage.
An example of such a current switching circuit is disclosed in Japanese Unexamined Patent Application Publication No. 2003-188465 (cf. FIG. 6). The circuit includes a current mirror 10, a current source (referred to hereinafter as a variable current source) 20, a pulse generator 30, a switch 40, and a superimposer (referred to hereinafter as a current energized circuit) 50 for reducing a pulse rising time. The current mirror 10 is composed of P-channel MOS transistors 11 and 12, and the MOS transistor 12 supplies to a laser diode LD the output current I2 which is proportional to the current I1 input to the MOS transistor 11. The variable current source 20 is composed of a variable voltage source 21 and an N-channel MOS transistor 22. The MOS transistor 22 is controlled by the variable voltage source 21 to regulate the current value of the current I1 to be supplied to the MOS transistor 11. The pulse generator 30 controls the switch 40 by a control signal S1 to supply the current I1 from the variable current source 20 to the MOS transistor 11. The current energized circuit 50 is composed of a one-shot circuit 51, an N-channel MOS transistor 52, and an N-channel MOS transistor 53. The MOS transistor 53 is controlled by the variable voltage source 21 to regulate the current value of the additional current I3 to be supplied to the MOS transistor 11. The one-shot circuit 51 controls the MOS transistor 52 by a control signal S2 to supply the additional current I3 from the MOS transistor 53 to the MOS transistor 11 at the rising edge of the output current I2.
The circuit of FIG. 6 has the variable voltage source 21 so as to control the intensity of the output current I2 in order to provide an appropriate output from the laser diode according to usage. Because the variable voltage source 21 is connected in common to the gates of the MOS transistor 22 and the MOS transistor 53, by the voltage settings of the variable voltage source 21 according to the intensity of the output current I2, the additional current I3 increases as the current I1 increases, and the additional current I3 decreases as the current I1 decreases.
In the circuit of FIG. 6, when turning on the MOS transistor 52, the additional current I3 turns ON and thereby the gate voltage of the MOS transistor 53 can be subject to fluctuations due to the parasitic capacitance between the drain and the gate of the MOS transistor 53. Consequently, upon changing the intensity of the output current I2, even if the set voltage of the variable voltage source 21 in accordance with the intensity of the output current I2 is applied to the gate of the MOS transistor 53, the MOS transistor 53 is not controlled by the set voltage of the variable voltage source 21 at the rising edge of the additional current I3, which causes overshoot or rounding to occur at the rising edge of the output current I2, thereby failing to supply the stable output current I2 to the laser diode LD.